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1.
A system of rational regime equations is developed for gravel-bed rivers with stable banks using the optimality theory (OT). The optimality theory is based on the premise that equilibrium river geometry is characterised by an optimum configuration, defined here as maximum sediment-transport efficiency. Theoretical dimensionless equations are derived for width, depth, slope, width/depth ratio, and meandering–braiding transition. Independent dimensionless variables comprise discharge, sediment concentration, and relative bank strength, μ′, which is defined as the ratio of the critical shear stresses for the bank and bed sediments. Discharge exponents and general form of the equations agree well with previously developed empirical relations. Relative bank strength, μ′, is used to parameterise the influence of riparian vegetation on bank strength and is evaluated by calibrating against observed width/depth ratio. Once calibrated, the hydraulic geometry of natural gravel rivers is well described by the theoretical equations, including discrimination between meandering and braiding channels. The results provide strong support for the assumption that equilibrium or regime river behavior is equivalent to an optimal state and underline the importance of bank strength and sediment load as controls on hydraulic geometry. 相似文献
2.
Bankfull channel width is a fundamental measure of stream size and a key parameter of interest for many applications in hydrology, fluvial geomorphology, and stream ecology. We developed downstream hydraulic geometry relationships for bankfull channel width w as a function of drainage area A, w = α Aβ, (DHGwA) for nine aggregate ecoregions comprising the conterminous United States using 1588 sites from the U.S. Environmental Protection Agency's National Wadeable Streams Assessment (WSA), including 1152 sites from a randomized probability survey sample. Sampled stream reaches ranged from 1 to 75 m in bankfull width and 1 to 10,000 km2 in drainage area. The DHGwA exponent β, which expresses the rate at which bankfull stream width scales with drainage area, fell into three distinct clusters ranging from 0.22 to 0.38. Width increases more rapidly with basin area in the humid Eastern Highlands (encompassing the Northern and Southern Appalachians and the Ozark Mountains) and the Upper Midwest (Great Lakes region) than for the West (both mountainous and xeric areas), the southeastern Coastal Plain, and the Northern Plains (the Dakotas and Montana). Stream width increases least rapidly with basin area in the Temperate Plains (cornbelt) and Southern Plains (Great Prairies) in the heartland. The coefficient of determination (r2) was least in the noncoastal plains (0.36–0.41) and greatest in the Appalachians and Upper Midwest (0.68–0.77). DHGwA equations differed between streams with dominantly fine bed material (silt/sand) and those with dominantly coarse bed material (gravel/cobble/boulder) in six of the nine analysis regions. Where DHGwA equations varied by sediment size, fine-bedded streams were consistently narrower than coarse-bedded streams. Within the Western Mountains ecoregion, where there were sufficient sites to develop DHGwA relationships at a finer spatial scale, α and β ranged from 1.23 to 3.79 and 0.23 to 0.40, respectively, with r2 > 0.50 for 10 of 13 subregions (range: 0.36 to 0.92). Enhanced DHG equations incorporating additional data for three landscape variables that can be derived from GIS—mean annual precipitation, elevation, and mean reach slope—significantly improved equation fit and predictive value in several regions, most notably the Western Mountains and the Temperate Plains. Channel width was also related to human disturbance. We examined the influence of human disturbance on channel width using several indices of local and basinwide disturbance. Contrary to our expectations, the data suggest that the dominant response of channel width to human disturbance in the United States is a reduction in bankfull width in streams with greater disturbance, particularly in the Western Mountains (where population density, road density, agricultural land use, and local riparian disturbance were all negatively related to channel width) and in the Appalachians and New England (where urban and agricultural land cover and riparian disturbance were all negatively associated with channel width). 相似文献
3.
This paper demonstrates the application of Terrestrial Laser Scanning (TLS) to determine the full population of grain roughness in gravel-bed rivers. The technique has the potential to completely replace the need for complex, time-consuming manual sampling methods. Using TLS, a total of 3.8 million data points (mean spacing 0.01 m) were retrieved from a gravel bar surface at Lambley on the River South Tyne, UK. Grain roughness was extracted through determination of twice the local standard deviation (2σz) of all the elevations in a 0.15 m radius moving window over the data cloud. 2σz values were then designated to each node on a 5 cm regular grid, allowing fine resolution DEMs to be produced, where the elevation is equivalent to the grain roughness height. Comparisons are made between TLS-derived grain roughness and grid-by-number sampling for eight 2 m2 patches on the bar surface. Strong relationships exist between percentiles from the population of 2σz heights with measured a-, b-, and c-axes, with the closest matches appearing for the c-axis. Although strong relationships exist between TLS-derived grain roughness (2σz), variations in the degree of burial, packing and imbrication, results in very different slope and intercept exponents. This highlights that conventional roughness measurement using gravel axis length should be used with caution as measured axes do not necessarily represent the actual extent to which the grain protrudes into the flow. The sampling error inherent in conventional sampling is also highlighted through undertaking Monte Carlo simulation on a population of 2000 clasts measured using the grid-by-number method and comparing this with the TLS-derived population of grain roughness heights. Underestimates of up to − 23% and overestimates of up to + 50% were found to occur when considering the D84, and − 20% and overestimates of up to + 36% were found to occur when considering the D50. 相似文献
4.
Late Quaternary slip across the Cañada David detachment has produced an extensive array of Quaternary scarps cutting alluvial-fans along nearly the entire length (~ 60 km) of the range-bounding detachment. Eight regional alluvial-fan surfaces (Q1 [youngest] to Q8 [oldest]) are defined and mapped along the entire Sierra el Mayor range-front. Terrestrial cosmogenic nuclide 10Be concentrations from individual boulders on alluvial-fan surfaces Q4 and Q7 yield surface exposure ages of 15.5 ± 2.2 ka and 204 ± 11 ka, respectively. Formation of the fans is probably tectonic, but their evolution is strongly moderated by climate, with surfaces developing as the hydrological conditions have changed in response to climate change on Milankovitch timescales. Systematic mapping reveals that the fault scarp array along active range-bounding faults in Sierras Cucapa and El Mayor can be divided into individual rupture zones, based on cross-cutting relationships with alluvial-fans. Quantitative morphological ages of the Laguna Salada fault-scarps, derived from linear diffusive degradation modeling, are consistent with the age of the scarps based on cross-cutting relationships. The weighted means of the maximum mass diffusivity constant for all scarps with offsets < 4 m is 0.051 and 0.066 m2/ka for the infinite and finite-slope solutions of the diffusion equation, respectively. This estimate is approximately an order of magnitude smaller than the lowest diffusivity constants documented in other regions and it probably reflects the extreme aridity and other microclimatic conditions that characterize the eastern margin of Laguna Salada. 相似文献
5.
The efficacy of extreme events is directly linked to the flood power and the total energy expended. The geomorphic effectiveness of floods is evaluated in terms of the distribution of stream power per unit boundary area (ω) over time, for three very large floods of the 20th Century in the Indian Peninsula. These floods stand out as outliers when compared with the peak floods per unit drainage area recorded elsewhere in the world. We used flood hydrographs and at-a-station hydraulic geometry equations, computed for the same gauging site or a nearby site, to construct approximately stream-power curves and to estimate the total energy expended by each flood. Critical unit stream power values necessary to entrain cobbles and boulders were estimated on the basis of empirical relationships for coarse sediment transport developed by Williams [Williams, G.P., 1983. Paleohydrological methods and some examples from Swedish fluvial environments. I. Cobble and boulder deposits. Geografiska Annaler 65A, 227–243.] in order to determine the geomorphological effectiveness of the floods. The estimates indicate that the minimum power per unit area values for all three floods were sufficiently high, and stream energy was above the threshold of boulder movement (90 W m− 2) for several tens of hours. The peak unit stream power values and the total energy expended during each flood were in the range of 290–325 W m− 2 and 65–160 × 106 J respectively. The average and peak flood powers were found to be higher or comparable to those estimated for extreme palaeo or modern floods on low-gradient, alluvial rivers. 相似文献
6.
In this study, an attempt has been made to evaluate the temporal variations in specific stream power and the total energy available for geomorphic work during the monsoon season for the Tapi River, in central India. Continuous daily discharge data (1978–1990), hydraulic geometry equations and the relationship between discharge and water surface slope were used to compute the daily specific stream power (ω) for the Savkheda gauging site in the lower Tapi Basin. The total amount of energy generated by all the monsoon flows was estimated by integrating the area under the ω-graph derived for the monsoon season.The analyses of the 13-year daily discharge data reveal that the average and maximum ω values range from 4–20 W m− 2, and 22–964 W m− 2 respectively. Specific stream power duration curve derived for the site shows that for 25% of the time the power per unit area is > 10 W m− 2. Furthermore, unit stream power was found to be above the Williams' [Williams, G.P., 1983. Paleohydrological methods and some examples from Swedish fluvial environments. I. Cobble and boulder deposits. Geografiska Annaler 65A, 227–243.] threshold of pebble-movement (1.5 W m− 2), cobble-movement (16 W m− 2) and boulder-movement (90 W m− 2) for 71%, 15% and 2% of the time, respectively. Computations further indicate that the total amount of energy generated by the flows during the monsoon season is in the range of 37 MJ (deficit monsoon years) to 256 MJ (excess monsoon and/or flood years). Large floods have one-third share in the total monsoon energy expenditure. In the absence of appropriate data on the yearwise geomorphic effects, the geomorphic work was evaluated in terms of the total suspended sediment load transported. The total monsoon sediment load is strongly related to the total monsoon energy. The results of the study indicate that the average flow competence and capacity are remarkably higher during wetter monsoon seasons and flood years than during the shorter and drier monsoon seasons.The present analyses demonstrate that the flows are geomorphically effective for a greater part of the monsoon season, except during the deficient monsoon years, and there is little doubt that large-magnitude floods are effective agents of geomorphic change in monsoonal rivers. 相似文献
7.
An expert-based approach was used to identify 10 morphological unit types within a reach of the gravel bed, regulated Yuba River, California, that is heavily utilized by spawning Chinook salmon (Oncorhynchus tshawytscha). Analysis of these units was carried out using two-dimensional hydrodynamic modeling, field-based geomorphic assessment, and detailed spawning surveying. Differently classified morphological units tended to exhibit discrete hydraulic signatures. In most cases, the Froude number adequately differentiated morphological units, but joint depth–velocity distributions proved the most effective hydraulic classification approach. Spawning activity was statistically differentiated at the mesoscale of the morphological unit. Salmon preferred lateral bar, riffle, and riffle entrance units. These units had moderately high velocity (unit median > 0.45 m s− 1) and low depth (unit median < 0.6 m), but each exhibited a unique joint depth–velocity distribution. A large proportion of redds (79%) were associated with conditions of convective flow acceleration at riffle and riffle entrance locations. In addition to reflecting microhabitat requirements of fish, it was proposed that the hydraulic segregation of preferred from avoided or tolerated morphological units was linked to the mutual association of specific hydraulic conditions with suitable caliber sediment that promotes the provision and maintenance of spawning habitat. 相似文献
8.
The sidewall effects of a wind tunnel on aeolian sand transport were investigated experimentally. A wind tunnel was used to conduct the experiments with a given channel height of 120 cm and varying widths (B) of 40, 60, 80, 100 and 120 cm. Both vertical profiles of wind velocity and sand mass flux were measured at different locations across the test section. The results show that the wind velocity with saltation first increases and then decreases to a minimum, from the sidewall to the central line of the wind tunnel. The discrepancy among wind velocities at different locations of the transverse section decreases with decreasing tunnel width. The wind friction velocity across the wind tunnel floor, with the exception of the region closest to the sidewalls, does not deviate strongly in wide wind tunnels from that along the central line, whereas it does vary in narrow tunnels. The sand mass fluxes, with the exception of some near-bed regions, are larger along the central line of the wind tunnel than they are at the quarter width location from the sidewall. Unlikely previously reported results, the dimensionless sand transport rate, Qg / (ρu3) (where Q is the total sand transport rate, g is the gravitational acceleration constant, ρ is the air density, and u is the wind friction velocity), first decreases and then increases with the dimensionless friction velocity, u / ut (where ut is the threshold friction velocity). The above differences may be attributed to the sidewall effects of the wind tunnel. A dimensionless parameter, FB = u / (gB)1/2, is defined to reflect the sidewall effects on aeolian sand transport. The flows with FB of 0.33 or less may be free from the sidewall effects of the wind tunnel and can ensure accurate saltation tunnel simulation. 相似文献
9.
To characterize the factors controlling pool shape, 30 different forced pools were created utilizing a 50% triangular constriction in a 0.5-m wide, 6-m long recirculating flume. Pools were scoured from an initial plane bed of sand with a d50 of 0.25 mm. Pool depth and length were measured and used as dependent variables in least-squares, multiple-regression analyses. Discharge, channel-bed gradient and energy slope were the independent variables. Additional linear-regression analyses were conducted with either pool depth or length and stream power. Results indicate that both pool depth and length are primarily a function of discharge. Channel-bed and energy slopes are also significantly related to pool length but are not significantly related to pool depth. Stream power is significantly related to both pool depth and length, but R2 values for pool depth versus discharge indicate stronger relations than those between pool depth and stream power. Observations on the type of geometric adjustment indicate that pools may minimize their rate of energy expenditure primarily through elongation. In contrast, pool depth appears to be more sensitive to the characteristics of the constrictions that create the forced pools. The results suggest that many field studies may suffer from cross-correlation problems. In particular, channel erodibility may exert a more dominant influence on pool geometry than hydraulic controls in many constriction-influenced channels. 相似文献
10.
Philip R. Kaufmann John M. Faustini David P. Larsen Mostafa A. Shirazi 《Geomorphology》2008,99(1-4):150-170
Quantitative regional assessments of streambed sedimentation and its likely causes are hampered because field investigations typically lack the requisite sample size, measurements, or precision for sound geomorphic and statistical interpretation. We adapted an index of relative bed stability (RBS) for data calculated from a national stream survey field protocol to enable general evaluation of bed stability and anthropogenic sedimentation in synoptic ecological surveys. RBS is the ratio of bed surface geometric mean particle diameter (Dgm) divided by estimated critical diameter (Dcbf) at bankfull flow, based on a modified Shield's criterion for incipient motion. Application of RBS to adequately depict bed stability in complex natural streams, however, has been limited because typical calculations of RBS do not explicitly account for reductions in bed shear stress that result from channel form roughness. We modified the index (RBS) to incorporate the reduction in bed shear stress available for sediment transport that results from the hydraulic resistance of large wood and longitudinal irregularities in channel dimensions (“form roughness”). Based on dimensional analysis, we derived an adjustment to bankfull shear stress by multiplying the bankfull hydraulic radius (Rbf) by the one-third power of the ratio of particle-derived resistance to total hydraulic resistance (Cp/Ct)1/3, where both resistances are empirically based calculations. We computed Cp using a Keulegan equation relating resistance to relative submergence of bed particles. We then derived an empirical equation to predict reach-scale hydraulic resistance Ct from thalweg mean depth, thalweg mean residual depth, and large wood volume based on field dye transit studies, in which total hydraulic resistance Ct was measured over a wide range of natural stream channel complexity, including manipulation of large wood volumes. We tested our estimates of Ct and RBS by applying them to data from a summer low flow probability sample of 104 wadeable stream reaches in the Coastal Ecoregion of Oregon and Washington, USA. Stream discharges calculated using these Ct estimates compared favorably with velocity–area measurements of discharge during summer low flow, and with the range of 1 to 2-year recurrence floods (scaled by drainage area) at U.S.Geological Survey gauged sites in the same region. Log [RBS] ranged from − 4.2 to + 0.98 in the survey region. Dgm ranged from silt to boulders, while estimated bankfull critical diameter, Dcbf, ranged from very fine gravel to large boulders. The median value of Dcbf (adjusted for form roughness influences) averaged 40% (inter quartile range 28 to 59%) of the unadjusted estimate Dcbf. Log[RBS] was consistently negatively related to human disturbances likely to produce excess sediment inputs or hydrologic alteration. Log [RBS] ranged from − 1.9 to + 0.5 in the streams within the lower quartile of human disturbance in their basin and riparian areas and was substantially lower (− 4.2 to − 1.1) in streams within the upper quartile of human disturbance. The synoptic survey methods and designs we used appear adequate to evaluate regional patterns in bed stability and sedimentation and their general relationship to human disturbances. Although the RBS concept also shows promise for evaluating sediment and bed stability in individual streams, our approach is relatively coarse, so site-specific assessments using these rapid field methods might prudently be confined to identifying severe cases of sedimentation or channel alteration. Greater confidence to discern subtle differences in site-specific assessments could be gained by calculating RBS using more precise field measurements of channel slope, bed particle size and bankfull dimensions, and by refining our adjustments for energy loss from channel form roughness. 相似文献
11.
The geomorphic function and characteristics of large woody debris in low gradient rivers, coastal Maine, USA 总被引：1，自引：0，他引：1
F.J. Magilligan K.H. Nislow G.B. Fisher J. Wright G. Mackey M. Laser 《Geomorphology》2008,97(3-4):467-482
The role, function, and importance of large woody debris (LWD) in rivers depend strongly on environmental context and land use history. The coastal watersheds of central and northern Maine, northeastern U.S., are characterized by low gradients, moderate topography, and minimal influence of mass wasting processes, along with a history of intensive commercial timber harvest. In spite of the ecological importance of these rivers, which contain the last wild populations of Atlantic salmon (Salmo salar) in the U.S., we know little about LWD distribution, dynamics, and function in these systems. We conducted a cross-basin analysis in seven coastal Maine watersheds, documenting the size, frequency, volume, position, and orientation of LWD, as well as the association between LWD, pool formation, and sediment storage. In conjunction with these LWD surveys, we conducted extensive riparian vegetation surveys. We observed very low LWD frequencies and volumes across the 60 km of rivers surveyed. Frequency of LWD ≥ 20 cm diameter ranged from 15–50 pieces km− 1 and wood volumes were commonly < 10–20 m3 km− 1. Moreover, most of this wood was located in the immediate low-flow channel zone, was oriented parallel to flow, and failed to span the stream channel. As a result, pool formation associated with LWD is generally lacking and < 20% of the wood was associated with sediment storage. Low LWD volumes are consistent with the relatively young riparian stands we observed, with the large majority of trees < 20 cm DBH. These results strongly reflect the legacy of intensive timber harvest and land clearing and suggest that the frequency and distribution of LWD may be considerably less than presettlement and/or future desired conditions. 相似文献
12.
Decoding temporal and spatial patterns of fault uplift using transient river long profiles 总被引：2，自引：1，他引：1
Alexander C. Whittaker Mikaël Attal Patience A. Cowie Gregory E. Tucker Gerald Roberts 《Geomorphology》2008,100(3-4):506-526
We present detailed observations of rivers crossing active normal faults in the Central Apennines, Italy, where excellent constraints exist on the temporal and spatial history of fault movement. We demonstrate that rivers with drainage areas > 10 km2 and crossing faults that have undergone an increase in throw rate within the last 1 My, have significant long-profile convexities. In contrast, channels that cross faults that have had a constant-slip rate for 3 My have concave-up profiles and have similar concavities and steepness indices to rivers that do not cross any active fault structures. This trend is consistent across the Central Apennines and cannot be explained by appeal to lithology or regional base level change. The data challenge the belief that active faulting must always be reflected in river profiles; instead, the long-profile convexities are best explained as a transient response of the river system to a change in tectonic uplift rate. Moreover, for these rivers we demonstrate that the height of the profile convexity, as measured from the fault, scales with the magnitude of the uplift rate increase on the fault; and we establish that this relationship holds for throw rate variation along strike for the same fault segment, as well as between faults. These findings are shown to be consistent with predictions of channel response to changing uplift rate rates using a detachment-limited fluvial erosion model, and they illustrate that analysis of the magnitude of profile convexities has considerable predictive potential for extracting tectonic information. We also demonstrate that the migration rate of the profile convexities varies from 1.5–10 mm/y, and is a function of the slip rate increase as well as the drainage area. This is consistent with n > 1 for the slope exponent in a classical detachment-limited stream-power erosion law, but could potentially be explained by incorporating an erosion threshold or an explicit role for sediment in enhancing erosion rates. Finally, we show that for rivers in extensional settings, where the response times to tectonic perturbation are long (in this case > 1 My), attempts to extract tectonic uplift rates from normalised steepness indices are likely to be flawed because topographic steady state has not yet been achieved. 相似文献
13.
Tracking of saltating sand trajectories over a flat surface embedded in an atmospheric boundary layer 总被引：4，自引：1，他引：3
Saltation is a major mechanism for the transport of soil particles. In the present study, we carried out wind tunnel tests to examine the saltating trajectories of two types of natural sand collected from a beach (diameter, d = 300–500 μm and 200–300 μm respectively) as well as sand from the Taklimakan desert (d = 100–125 μm) in an atmospheric boundary layer. Consecutive images of saltating particles were recorded using a high-speed digital camera at a rate of 2000 fps with a spatial resolution of 1024 × 1024 pixels. The high temporal resolution of the acquired images enabled us to study the particle motion very close to the surface. The saltating particle trajectories were reconstructed from consecutive images, and the physical quantities characterizing the initial and final stages of the particle flight in the windward direction at friction velocities of about 10%–25% above the threshold friction velocity (u / ut = 1.11–1.26) were analyzed statistically. In addition, the transverse deviation of the saltating particles from the main streamwise direction was evaluated. The results shed new light on the complicated motions involved in sand saltation and should prove useful in the evaluation and formulation of theoretical models. 相似文献
14.
《Basin Research》2018,30(2):217-236
This study documents the seismic expression of the conduits underlying over 350 mud volcanoes that were erupted in an area of the western Nile Cone in the past 5.3 Myr. The study is based on a c. 4300 km2 3D seismic survey. The conduits are interpreted to transect the >1000‐m‐thick Messinian Evaporite succession, demonstrating that the eruptive process is sufficiently dynamic to breach the formidable seal represented by the evaporites. The mud volcano conduits are remarkably similar in geometry and seismic characteristics to many previously described examples of fluid escape pipes. They are vertical to subvertical structures with a crudely cylindrical geometry, but that can either widen or narrow upwards towards their upper terminations in the mud volcano edifices. Imaging at depth within the Messinian Evaporites and pre‐evaporite successions is more uncertain, but direct sampling of mud from surface volcanoes suggests a pre‐Messinian source, confirming the seismic interpretation that they root within presalt stratigraphy. A conceptual model for the genesis of these mud volcano conduits through salt is proposed, for which hydraulic fracturing is driven by high overpressures that developed in the presalt source stratigraphy as a response to the Messinian Salinity Crisis. Dissolution and removal of evaporites resulting in fracturing and collapse via a stoping mechanism is a slow process by comparison to hydraulic fracturing but is argued to potentially contribute to conduit formation. The analysis presented here demonstrates the potential for bypassing a >1‐km‐thick unit of sealing evaporites via focused fluid and sediment mobilisation from deeper overpressured cells in other salt basins worldwide, and has significant implications for hydrocarbon exploration, CO2 sequestration and nuclear waste disposal. 相似文献
15.
Qingjie Han Jianjun Qu Kecun Zhang Ruiping Zu Qinghe Niu Kongtai Liao 《Geomorphology》2009,104(3-4):230-237
Wind erosion has major impacts on dune growth, desertification, and architecture on sea coasts. The deflation threshold shear velocity is a crucial parameter in predicting erosion, and surface moisture greatly affects this threshold and thus sand stability. Wind tunnel studies have shown that reduced moisture contents decrease entrainment thresholds and increase wind erosion, but field and wind tunnel test data is lacking for tropical humid coastal areas. In this study, we investigated the influence of surface moisture contents (at 1 mm depth) on sand entrainment and erosion using tropical humid coastal sands from southern China. Shear velocities were deduced from velocity profiles above the sand. The threshold shear velocity increased linearly with increasing ln100M (M, gravimetric moisture content). The increase was steepest below a moisture content of 0.0124 (i.e., at M1.5, the moisture content in the sand at a matric potential of − 1.5 MPa). We compared several popular models that predict threshold shear velocity of moisture sediment, and found substantial differences between their predicted results. At a surface moisture content of 0.0124, the predicted increase in the wet threshold shear velocity compared with the dry threshold shear velocity ranged from 34% to 195%. The empirical model of Chepil and Selah simulated the data well for M < 0.0062 (i.e., 0.5M1.5), whereas Belly's empirical model simulated the data best for M > 0.0062. Wind erosion modulus increased with increasing effective wind velocity following a power function with a positive exponent at all moisture contents, but decreased with increasing surface moisture content following a power function with a negative exponent. When wind speed and moisture content varied simultaneously, wind erosion modulus was proportional to the 0.73 power of effective wind velocity, but inversely proportional to the 1.48 power of M. The increase in resistance to erosion at low moisture contents probably results from cohesive forces in the water films surrounding the sand particles. At a moisture content near M1.5, wind erosion ceases nearly for all wind velocities that we tested. 相似文献
16.
Carbon accumulation, nitrogen and phosphorus use efficiency of Sophora davidii seedlings in response to nitrogen supply and water stress 总被引：1，自引：0，他引：1
A greenhouse experiment was conducted to explore whether additional nitrogen (N) supply could enhance carbon (C) accumulation, and phosphorus (P) use efficiency (NUEP) of Sophora davidii seedlings under dry conditions. Two-month-old seedlings were subjected to a completely randomized design with three water (80, 40 and 20% water field capacity (FC)) and three N supply (N0: 0, Nl: 92 and Nh: 184 mg N kg−1 soil) regimes. Water stress decreased C, N and P accumulation, NUEP, N and P uptake efficiency (NUtEN and NUtEP) regardless of N supply. The S. davidii seedlings exhibited strong responses to N supply, but the responses were not consistent with the various N supply levels. Nl increased C, N and P accumulation, and improved NUEP, NUtEN and NUtEP in the same water treatment. In contrast, Nh did few or even depress effects on C, N and P accumulation, and NUEP, although NUtEN and NUtEP increased with Nh in the same water treatment. Even so, NUEN decreased with increase of N supply in the same water treatment. The results suggested that appropriate or low N supply should be recommended for S. davidii seedling establishment in dry environment by improving C accumulation and NUEP. 相似文献
17.
Flux and fate of Yangtze River sediment delivered to the East China Sea 总被引：57，自引：0，他引：57
J.P. Liu K.H. Xu A.C. Li J.D. Milliman D.M. Velozzi S.B. Xiao Z.S. Yang 《Geomorphology》2007,85(3-4):208
Numerous cores and dating show the Yangtze River has accumulated about 1.16 × 1012 t sediment in its delta plain and proximal subaqueous delta during Holocene. High-resolution seismic profiling and coring in the southern East China Sea during 2003 and 2004 cruises has revealed an elongated ( 800 km) distal subaqueous mud wedge extending from the Yangtze River mouth southward off the Zhejiang and Fujian coasts into the Taiwan Strait. Overlying what appears to be a transgressive sand layer, this distal clinoform thins offshore, from 40 m thickness between the 20 and 30 m water depth to < 1–2 m between 60 and 90 m water depth, corresponding to an across shelf distance of less than 100 km. Total volume of this distal mud wedge is about 4.5 × 1011 m3, equivalent to 5.4 × 1011 t of sediment. Most of the sediment in this mud wedge comes from the Yangtze River, with some input presumably coming from local smaller rivers. Thus, the total Yangtze-derived sediments accumulated in its deltaic system and East China Sea inner shelf have amounted to about 1.7 × 1012 t. Preliminary analyses suggest this longshore and across-shelf transported clinoform mainly formed in the past 7000 yrs after postglacial sea level reached its mid-Holocene highstand, and after re-intensification of the Chinese longshore current system. Sedimentation accumulation apparently increased around 2000 yrs BP, reflecting the evolution of the Yangtze estuary and increased land erosion due to human activities, such as farming and deforestation. The southward-flowing China Coastal Current, the northward-flowing Taiwan Warm Current, and the Kuroshio Current appear to have played critical roles in transporting and trapping most of Yangtze-derived materials in the inner shelf, and hence preventing the sediment escape into the deep ocean. 相似文献
18.
Quanzhou Gao Zhen Tao Xiakun Huang Ling Nan Kefu Yu Zhengang Wang 《Geomorphology》2009,106(3-4):324-332
Monthly samples of riverine water were collected and analyzed for the concentrations of major ions (Ca2+, Mg2+, K+, Na+, HCO3−, SO42−, Cl−, NO3−), dissolved silicon, and total dissolved solids (TDS) at Wuzhou hydrological station located between the middle and lower reaches of the Xijiang River (XJR) from March 2005 to April 2006. More frequent sampling and analysis were carried out during the catastrophic flooding in June 2005. Stoichiometric analysis was applied for tracing sources of major ions and estimating CO2 consumption from the chemical weathering of rocks. The results demonstrate that the chemical weathering of carbonate and silicate rocks within the drainage basin is the main source of the dissolved chemical substances in the XJR. Some 81.20% of the riverine cations originated from the chemical weathering processes induced by carbonic acid, 11.32% by sulfuric acid, and the other 7.48% from the dissolution of gypsum and precipitates of sea salts within the drainage basin. The CO2 flux consumed by the rock chemical weathering within the XJR basin is 2.37 × 1011 mol y− 1, of which 0.64 × 1011 mol y− 1 results from silicate rock chemical weathering, and 1.73 × 1011 mol y− 1 results from carbonate rock chemical weathering. The CO2 consumption comprises 0.38 × 1011 mol during the 9-d catastrophic flooding. The CO2 consumption from rock chemical weathering in humid subtropical zones regulates atmospheric CO2 level and constitutes a significant part of the global carbon budget. The carbon sink potential of rock chemical weathering processes in the humid subtropical zones deserves extra attention. 相似文献
19.
The distribution and age of large (> 0.1 km2) Pliocene to recent rockslides in the Chilean Cordillera Principal (32–34.5 S), the Southern Central Andes, has been analyzed to determine the rockslide triggering mechanisms and impact on regional landscape evolution. Most of the rockslides appear in the western Cordillera Principal and cluster along major geological structures. Variographic analyses show spatial correlation between rockslides, geological structures and shallow seismicity. A relative chronosequence was calibrated with existing 14C and 40Ar/39Ar dates and new cosmogenic nuclide exposure ages for selected rockslides. Rockslide-induced sediment yield was estimated with empirical relations for rockslide area distributions. Throughout the Quaternary, rockslides have delivered sediment to streams at rates equivalent to denudation rates of 0.10 ±0.06 mm a− 1, while estimates using short term (20 a) seismicity records are 0.3− 0.2+ 0.6 mm a− 1. The estimates of sediment transfer and the spatial distribution of rockslides reflect a landscape in which tectonic and geological controls on denudation are more significant than climate. 相似文献
20.
Initiation conditions for debris flows generated by runoff at Chalk Cliffs, central Colorado 总被引：7，自引：1，他引：6
We have monitored initiation conditions for six debris flows between May 2004 and July 2006 in a 0.3 km2 drainage basin at Chalk Cliffs; a band of hydrothermally-altered quartz monzonite in central Colorado. Debris flows were initiated by water runoff from colluvium and bedrock that entrained sediment from rills and channels with slopes ranging from about 14° to 45°. The availability of channel material is essentially unlimited because of thick channel fill and refilling following debris flows by rock fall and dry ravel processes. Rainfall exceeding I = 6.61(D)− 0.77, where I is rainfall intensity (mm/h), and D is duration (h), was required for the initiation of debris flows in the drainage basin. The approximate minimum runoff discharge from the surface of bedrock required to initiate debris flows in the channels was 0.15 m3/s. Colluvium in the basin was unsaturated immediately prior to (antecedent) and during debris flows. Antecedent, volumetric moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 4–9%, and 4–7%, respectively. During debris flows, peak moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 10–20%, and 4–12%, respectively. Channel sediment at a depth of 45 cm was unsaturated before and during debris flows; antecedent moisture ranged from 20–22%, and peak moisture ranged from 24–38%. Although we have no measurements from shallow rill or channel sediment, we infer that it was unsaturated before debris flows, and saturated by surface-water runoff during debris flows.Our results allow us to make the following general statements with regard to debris flows generated by runoff in semi-arid to arid mountainous regions: 1) high antecedent moisture levels in hillslope and channel sediment are not required for the initiation of debris flows by runoff, 2) locations of entrainment of sediment by successive runoff events can vary within a basin as a function of variations in the thickness of existing channel fill and the rate of replenishment of channel fill by rock fall and dry ravel processes following debris flows, and 3) rainfall and simulated surface-water discharge thresholds can be useful in understanding and predicting debris flows generated by runoff and sediment entrainment. 相似文献